Public switched telephone networks in the United States utilize digital transmission systems, commonly known as digital carrier systems. The current nature of such systems is a result of historical development. Digital carrier technology was initially deployed to support trunks between metropolitan central offices or COs (1962), and then in the feeder plant connecting the user to the CO (1973). Since the early 1980s, digital multiplex technology has become directly available to end users in the form of "T1" private lines, and lately as "fractional T1" private lines. ISDN may be considered a companion digital technology applicable to the distribution plant.
Initially, carrier technology was intimately connected with digital voice transmission and PCM (pulse code modulation) methods. With newer non-PCM voice digitization schemes, and the extensive commercial applications of T1 in the high-speed environment, a description of digital carrier systems no longer needs to be subordinated as a mere feature of PCM. However, the peculiarities these digital facilities inherited from their voice ancestry cannot be avoided. It is therefore desirable to deal with the multiplexing schemes employed in T1, restrictions on the user signal imposed by those schemes, typical CPE and carrier equipment, schemes for higher digital rates, and evolving transmission approaches for broadband ISDN.
In the early days of telecommunication, a medium such as copper wire carried a single information channel. Because of economic reasons, both in terms of construction costs and in terms of materials, new ways of packing multiple channels onto a single physical link were needed. The resulting system is referred to as a carrier system, or simply a carrier. Carrier systems may be analog or digital. Analog systems are decreasing in importance, and are now a small fraction of the total number of carrier systems in the public switched network.
The first digital system introduced into the public network was the so-called "T-carrier" system, which utilized twisted pair wire. Digital signals are now transmitted from one location to another by facilities using a multitude of media, including paired cable, coaxial cable, radio, optical fibers, and satellite. A carrier system consists of a transmission component, a user interface component, and a user termination equipment component.
The transmission component is a transmission system carrying multiple channels, which in turn entails multiplexer equipment and a transmission link. T-carriers in a strict telephonic sense, are copper-based digital facilities that carry 24, 96, 672, or 4032 simultaneous PCM-coded voice or voice band data channels, operating at 64 kb/s each (the T1, T2, T3, and T4 systems, respectively). In reality, many carrier systems today are based on fiber transmission. An alternate nomenclature used in describing generic digital carrier systems is DS1, DS2, DS3, and DS4, where DS stands for digital signal. In particular, "DS" refers to the coding format used to transmit the information over the carrier system. Carrier termination equipment includes telephone company (Telco) equipment such as channel banks, transcoder, and digital cross-connect systems; and CPE, such as T1 multiplexers and PBXs (private branch exchanges). Generally, CPE termination equipment must be interfaced to the carrier system via a channel service unit (CSU). T-carrier systems use time division multiplexing (TDM).
The traditional TDM hierarchy is described as DS level 0 through DS level 4. The 0 to 4 kHz nominal voiceband channels are first converted to a digital streams by PCM analog-to-digital techniques, and then multiplexed onto higher bit streams. Each of the individual digitized 64 Kb/s channels is referred to as a DS0 level. In the United States, the traditional digital hierarchy uses 1.544 Mb/s for 24 channels, also called a digroup (for digital group), 3.152 Mb/s for 48 channels, 6.312 Mb/s for 96 channels, 44.736 Mb/s for 672 channels, and 274.176 Mb/s for 4032 channels. As stated, these are commonly called T1, T2, T3, and T4, respectively. These digital streams produced by channel banks and other multiplex equipment are independent of the target transmission media; in fact, in an end-to-end circuit, many different types of media may be encountered.
The output of a T1 multiplexer or a channel bank at the DS1 level may be placed on copper facilities, which are designated as T1 systems; the output of a channel bank at the DS2 level may be placed on copper facilities, designated as T2; the output of a DS3 system may be placed on a fiber optic facility, designated FT3. Thus, a T1 system must use a DS1 signal format; but a DS1 signal need not use a T1 facility.
The rates corresponding to the TDM hierarchy were initially derived based on the information carrying capacity of copper wires and the spacing of manholes where repeaters could be located.
Referring to FIG. 1 there is shown a simplified block diagram of a typical electronic program controlled switch which may be used as signal point (SP) or signal switching point (SSP) type switching offices in public switched telephone networks operated by telephone companies (Telco's). As illustrated, the switch includes a number of different types of modules. In particular, the illustrated switch includes interface modules 51 (only two of which are shown), a communications module 53 and an administrative module 55.
The interface modules 51 each include a number of interface units 0 to n. The interface units terminate lines from subscribers' stations, trunks, T1 carrier facilities, etc. Where the interfaced circuit is analog, for example a subscriber loop, the interface unit will provide analog to digital conversion and digital to analog conversion. The interface modules for the analog lines also include dial pulse detectors and dual tone multifrequncy (DTMF) detectors. Alternatively, the lines or trunks may use digital protocols such as T1 or ISDN. Each interface module 51 also includes a digital service unit (not shown) which is used to generate call progress tones.
Each interface module 51 includes, in addition to the noted interface units, a duplex microprocessor based module controller and a duplex time slot interchange, referred to as a TSI in the drawing. Digital words representative of voice information are transferred in two directions between interface units via the time slot interchange (intramodule call connections) or transmitted in two directions through the network control and timing links to the time multiplexed switch 57 and thence to another interface module (intermodule call connection).
The communication module 53 includes the time multiplexed switch 57 and a message switch 59. The time multiplexed switch 57 provides time division transfer of digital voice data packets between voice channels of the interface modules 51 and transfers data messages between the interface modules. The message switch 59 interfaces the administrative module 55 to the time multiplexed switch 57, so as to provide a route through the time multiplexed switch permitting two-way transfer of control related messages between the interface modules 51 and the administrative module 55. In addition, the message switch 59 terminates special data links, for example a link for receiving a synchronization carrier used to maintain digital synchronism.
The administrative module 55 includes an administrative module processor 61, which is a computer equipped with disc storage 63, for overall control of operations of the switching office. The administrative module processor 61 communicates with the interface modules 51 through the communication module 55. The administrative module 55 also includes one or more input/output (I/O) processors 65 providing interfaces to terminal devices for technicians such as shown at 66 in the drawing and data links to operations systems for traffic, billing, maintenance data, etc. A common channel interoffice switching (CCIS) terminal 73 and an associated data unit 71 provide a signaling link between the administrative module processor 61 and an STP of the SS7 signaling network, for facilitating call processing signal communications with other central offices (COs) and with one or more of the SCPs and/or the ISCP 17.
As illustrated in FIG. 1, the administrative module 55 also includes a call store 67 and a program store 69. Although shown as separate elements for convenience, these are typically implemented as memory elements within the computer serving as the administrative module processor 61. For each call in progress, the call store 67 stores translation information retrieved from disc storage 63 together with routing information and any temporary information needed for processing the call. For example, for a switch based Centrex type service, the call store 67 would receive and store extension number translation information for the business customer corresponding to an off-hook line initiating a call. The program store 69 stores program instructions which direct operations of the computer serving as the administrative module processor.
Referring to the interface modules 51, a number of these are provided in a typical program controlled telecommunications network switch, such as 5ESS switches manufactured by AT&T, and equivalent switches of other manufacturers. The devices labeled as "units" are normally line or interface cards. These connect to module control and TSI (time slot interchange or time division multiplexed switch) shown in FIG. 1. Those in turn are interconnected to each other through what may be called a higher level time division multiplexed switch shown at 57. The line and trunk interface units connect to POTS (plain old telephone service) or ISDN (Integrated service digital network) lines. The trunk units connect to T1 or higher capacity lines or trunks.
On the back or right side of these units in FIG. 1 the signals comprise digital variations of 64 K/s time slot signaling. In effect this imposes units or slots of 64 K/s as the minimum bandwidth and it is not ordinarily possible to get below this 64 K/s as the unit of bandwidth. The 64 K/s derives from the use of T1 digital carrier. As previously discussed, the T1 digital carrier system is based on digitizing a 4000 Hz signal by sampling at an 8000 times per second rate to obtain 8 bits per sample or 64,000 bits per channel. The resulting PAM (pulse amplitude modulated) signal is encoded in the channel bank and merged with 23 other voice channels to form the 24 DO, DS1 1.544 mb/s T1 carrier. If one subscriber needs 4 K/s for a good voice quality circuit using compression, the other 60 K/s of that D0 channel is unused. If a T1 line is tied up with a conversation at 64 K/s bandwidth is being wasted because the largest amount of that 64 K/s bandwidth is not being used.